Chiral recognition of amethopterin enantiomers by the reduced folate carrier in Caco-2 cells

Positively charged helical chain-modified stimuli-responsive nanoassembly capable of targeted drug delivery and photoacoustic imaging-guided chemo-photothermal synergistic therapy

Tumor targeted size-switchable CPT/IR780@H30-PCL-PPI(L−)/PEI(–COOH/FA) nanoassembly with a “pomegranate” construction was designed, which could efficiently expand the penetration depth and accelerate the cell internalization.

Effect of epigallocatechin gallate on drug transport mediated by the proton-coupled folate transporter

Folic acid (FA) is a water-soluble vitamin, and orally ingested FA is absorbed from the small intestine by the proton-coupled folate transporter (PCFT). In the present study, we investigated whether epigallocatechin gallate (EGCG), one of the tea catechins, affects the transport of FA by PCFT. EGCG inhibited the uptake of FA into Caco-2 cells and human PCFT-expressing HEK293 cells (PCFT-HEK293 cells). The initial rate of uptake of FA into PCFT-HEK293 cells followed Michaelis-Menten kinetics (K(m) = 1.9 µM). Dixon plots revealed that PCFT-mediated FA uptake was competitively inhibited by EGCG (K(i) ≒ 9 µM). The uptake of the PCFT substrate methotrexate (MTX) was competitively inhibited by EGCG as well (K(i) ≒ 15 µM). In conclusion, it is suggested that when FA or MTX is ingested with tea, it is likely that the intestinal absorption of these compounds by PCFT is inhibited, which could result in insufficient efficacy.

Generation of a luciferase-based reporter for CHH and CG DNA methylation in Arabidopsis thaliana

Background

DNA methylation ensures genome integrity and regulates gene expression in diverse eukaryotes. In Arabidopsis, methylation occurs in three sequence contexts: CG, CHG and CHH. The initial establishment of DNA methylation at all three sequence contexts occurs through a process known as RNA-directed DNA methylation (RdDM), in which small RNAs bound by Argonaute4 (AGO4) guide DNA methylation at homologous loci through the de novo methyltransferase DRM2. Once established, DNA methylation at each of the three sequence contexts is maintained through different mechanisms. Although some players involved in RdDM and maintenance methylation have been identified, the underlying molecular mechanisms are not fully understood. To aid the comprehensive identification of players in DNA methylation, we generated a transgenic reporter system that permits genetic and chemical genetic screens in Arabidopsis.

Results

A dual 35S promoter (d35S) driven luciferase (LUC) reporter was introduced into Arabidopsis and LUCL, a line with a low basal level of luciferase activity, was obtained. LUCL was found to be a multi-copy, single-insertion transgene that contains methylated cytosines in CG, CHG and CHH contexts, with the highest methylation in the CG context. Methylation was present throughout the promoter and LUC coding region. Treatment with an inhibitor of cytosine methylation de-repressed luciferase activity. A mutation in MET1, which encodes the CG maintenance methyltransferase, drastically reduced CG methylation and de-repressed LUC expression. Mutations in AGO4 and DRM2 also de-repressed LUC expression, albeit to a smaller extent than loss of MET1. Using LUCL as a reporter line, we performed a chemical screen for compounds that de-repress LUC expression, and identified a chemical, methotrexate, known to be involved in biogenesis of the methyl donor.

Conclusion

We developed a luciferase-based reporter system, LUCL, which reports both RdDM and CG maintenance methylation in Arabidopsis. The low basal level of LUCL expression provides an easy readout in genetic and chemical genetic screens that will dissect the mechanisms of RdDM and methylation maintenance.

A re-evaluation of the role of hCTR1, the human high-affinity copper transporter, in platinum-drug entry into human cells

Cisplatin (cDDP) is an anticancer drug used in a number of malignancies, including testicular, ovarian, cervical, bladder, lung, head, and neck cancers. Its use is limited by the development of resistance, often rationalized via effects on cellular uptake. It has been claimed that human copper transporter 1 (hCTR1), the human high-affinity copper transporter, is the major entry pathway for cDDP and related drugs via a mechanism that mimics copper. This is an unexpected property of hCTR1, a highly selective copper (I) transporter. We compared the uptake rates of copper with cDDP (and several analogs) into human embryonic kidney 293 cells overexpressing wild-type or mutant hCTR1, mouse embryonic fibroblasts that do or do not express CTR1, and human ovarian tumor cells that are sensitive or resistant to cDDP. We have also compared the effects of extracellular copper, which causes regulatory endocytosis of hCTR1, to those of cDDP. We confirm the correlation between higher hCTR1 levels and higher platinum drug uptake in tumor cells sensitive to the drug. However, we show that hCTR1 is not the major entry route of platinum drugs, and that the copper transporter is not internalized in response to extracellular drug. Our data suggest the major entry pathway for platinum drugs is not saturable at relevant concentrations and not protein-mediated. Clinical trials have been initiated that depend upon regulating membrane levels of hCTR1. If reduced drug uptake is a major factor in resistance, hCTR1 is unlikely to be a productive target in attempts to enhance efficacy, although the proteins involved in copper homeostasis may play a role.

Enantioselective uptake of fexofenadine by Caco-2 cells as model intestinal epithelial cells

OBJECTIVES

Fexofenadine contains a chiral carbon in its chemical structure and is orally administered as a racemic mixture. This study evaluated the selective uptake of fexofenadine enantiomers by Caco-2 cells as a model of intestinal epithelial cells.

METHODS

R(+)-fexofenadine or S(-)-fexofenadine was applied to Caco-2 cells, followed by incubation. After incubation, the amounts of fexofenadine enantiomers in cells were determined. The kinetic parameters for the uptake of fexofenadine enantiomers by Caco-2 cells were estimated using the Michaelis-Menten equation.

KEY FINDINGS

The transporter-mediated uptake rate of R(+)-fexofenadine was 1.7-fold higher than that of S(-)-fexofenadine. The difference in transporter-mediated R(+)-fexofenadine and S(-)-fexofenadine uptake was completely diminished under ATP-depleted conditions and in the presence of organic anion transporter peptide (OATP) inhibitors. Also, a Dixon plot showed that each fexofenadine enantiomer was competitively inhibited by the other enantiomer. The ratio of R(+)-fexofenadine uptake to S(-)-fexofenadine uptake in the case of a racemic mixture was higher than that in the case of a single enantiomer.

CONCLUSION

This study suggested that the selective absorption of fexofenadine enantiomers by intestinal epithelial cells might have been due to the selective uptake mediated by OATPs and that the difference in intestinal absorption was enhanced with a racemic mixture.

Intestinal transport of aminopterin enantiomers in dogs and humans with psoriasis is stereoselective: evidence for a mechanism involving the proton-coupled folate transporter

N-[4-[[(2,4-diamino-6-pterdinyl)methyl]amino]benzoyl]-L/D-glutamic acid (L/D-AMT) is an investigational drug in phase 1 clinical development that consists of the L-and D-enantiomers of aminopterin (AMT). L/D-AMT is obtained from a novel process for making the L-enantiomer (L-AMT), a potent oral antiinflammatory agent. The purpose of these studies was to characterize oral uptake and safety in the dog and human of each enantiomer alone and in combination and provide in vitro evidence for a mechanism of intestinal absorption. This is the first report of L /D-AMT in humans. In dogs (n = 40) orally dosed with L-AMT or D-AMT absorption was stereoselective for the L-enantiomer (6- to 12-fold larger peak plasma concentration after oral administration and area under the plasma concentration-time curve at 0-4 h; p < 0.001). D-AMT was not toxic at the maximal dose tested (82.5 mg/kg), which was 100-fold larger than the maximal nonlethal L-AMT dose (0.8 mg/kg). Dogs (n = 10) and humans with psoriasis (n = 21) orally administered L-AMT and L /D-AMT at the same L-enantiomer dose resulted in stereoselective absorption (absent D-enantiomer in plasma), bioequivalent L-enantiomer pharmacokinetics, and equivalent safety. Thus, the D-enantiomer in L/D-AMT did not perturb L-enantiomer absorption or alter the safety of L-AMT. In vitro uptake by the human proton-coupled folate transporter (PCFT) demonstrated minimal transport of D-AMT compared with L-AMT, mirroring the in vivo findings. Enantiomer selectivity by PCFT was attributable almost entirely to decreased binding affinity rather than changes in transport rate. Collectively, our results demonstrate a strong in vitro-in vivo correlation implicating stereoselective transport by PCFT as the mechanism underlying stereoselective absorption observed in vivo.

Stereoselective transport of amethopterin enantiomers by the proton-coupled folate transporter

Stereoselective transport of methotrexate (L-amethopterin, L-MTX) and its antipode (D-amethopterin, D-MTX) by the proton-coupled folate transporter (PCFT) was examined using PCFT-expressing HEK293 cells (PCFT-HEK293 cells). Uptake of both L-MTX and D-MTX was pH-dependent and decreased with an increase in the extracellular pH from 5.0 to 7.4. The initial uptake rate of L-MTX into PCFT-HEK293 cells followed Michaelis-Menten kinetics with a K(m) value of approximately 5.0 microM. Dixon plots revealed that L-MTX uptake was inhibited competitively by unlabeled L-MTX, D-MTX, and folic acid (FA), with K(i) values of approximately 3.6, 180, and 2.1 microM, respectively. The initial uptake rate of D-MTX into PCFT-HEK293 cells also followed Michaelis-Menten kinetics with a K(m) value of 211 microM. The V(max) value of D-MTX was similar to that of L-MTX. The present study revealed that the transport of MTX enantiomers by PCFT is highly stereoselective with the uptake clearance of L-MTX being approximately 40-fold greater than that of D-MTX. It was also revealed that this high stereoselectivity results from the difference in K(m) values, and not V(max) values, between the enantiomers. The observed stereoselectivity was consistent with the differences in the intestinal absorption of MTX enantiomers in humans.

Reliability of inhibition models to correctly identify type of inhibition

PURPOSE

Type of inhibition (e.g. competitive, noncompetitive) is frequently evaluated to understand transporter structure/function relationships, but reliability of nonlinear regression to correctly identify inhibition type has not been assessed. The purpose was to assess the ability of nonlinear regression to correctly identify inhibition type.

METHODS

This aim was pursued through three objectives that compared the competitive, noncompetitive, and uncompetitive inhibition models to best fit simulated competitive and noncompetitive data. The first objective involved conventional inhibition data and entailed simulated data for the common situation where substrate concentration was fixed at a single level but inhibitor concentration varied. The second objective involved Dixon-type data where both substrate and inhibitor concentrations varied. A third objective involved nonconventional inhibition data, where substrate concentration was varied and inhibitor was fixed at a single concentration. Experimental data were also examined.

RESULTS

Nonlinear regression performed poorly in identifying the correct inhibition model for conventional inhibition data, but performed moderately well for Dixon-type data. Interestingly, nonlinear regression performed well for nonconventional inhibition data, particularly at higher inhibitor concentrations. Experimental data support simulation findings.

CONCLUSIONS

Conventional inhibition data is a poor basis to determine inhibition type, while Dixon-type data affords modest success. Nonconventional inhibition data merits further consideration.

Chiral toxicology: it's the same thing...only different

Chiral substances possess a unique architecture such that, despite sharing identical molecular formulas, atom-to-atom linkages, and bonding distances, they cannot be superimposed. Thus, in the environment of living systems, where specific structure-activity relationships may be required for effect (e.g., enzymes, receptors, transporters, and DNA), the physiochemical and biochemical properties of racemic mixtures and individual stereoisomers can differ significantly. In drug development, enantiomeric selection to maximize clinical effects or mitigate drug toxicity has yielded both success and failure. Further complicating genetic polymorphisms in drug disposition, stereoselective metabolism of chiral compounds can additionally influence pharmacokinetics, pharmacodynamics, and toxicity. Optically pure pharmaceuticals may undergo racemization in vivo, negating single enantiomer benefits or inducing unexpected effects. Appropriate chiral antidotes must be selected for therapeutic benefit and to minimize adverse events. Enantiomers may possess different carcinogenicity and teratogenicity. Environmental toxicology provides several examples in which compound bioaccumulation, persistence, and toxicity show chiral dependence. In forensic toxicology, chiral analysis has been applied to illicit drug preparations and biological specimens, with the potential to assist in determination of cause of death and aid in the correct interpretation of substance abuse and "doping" screens. Adrenergic agonists and antagonist, nonsteroidal anti-inflammatory agents, SSRIs, opioids, warfarin, valproate, thalidomide, retinoic acid, N-acetylcysteine, carnitine, penicillamine, leucovorin, glucarpidase, pesticides, polychlorinated biphenyls, phenylethylamines, and additional compounds will be discussed to illustrate important concepts in "chiral toxicology."

Simultaneous determination of methotrexate and its polyglutamate metabolites in Caco-2 cells by liquid chromatography-tandem mass spectrometry

In normal and malignant human cells, the folate antagonist methotrexate (MTX) is converted to a series of polyglutamates (MTXGlu(n), n=2-5) which play a role in its therapeutic efficacy. Here we report an assay to determine MTX and MTXGlu(n) in Caco-2 cells exposed to MTX. After a simple protein precipitation step, cell homogenates (2 x 10(6) cells) were analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) using aminopterin as internal standard. Separation was by reversed phase HPLC on a C8 column using gradient elution with 0.1% formic acid and acetonitrile. Detection was by electrospray ionization in the positive ion mode followed by multiple reaction monitoring of the transitions of the [M+H](+) ions of MTX and MTXGlu(n) to their common product ion at m/z 308.2 and of aminopterin at m/z 441.3-->294.2. Calibration curves for all analytes were linear in the range 2-250 nM (r(2)>0.996). Intra- and inter-day precisions (as coefficient of variation) were 3.4-15.1% and 4.3-18.4%, respectively with corresponding accuracies (as relative error) of -3.6 to +6.6% and -5.5 to +7.5%, respectively. Recoveries were in the range 60+/-4 to 108+/-13%. It was found that MTX undergoes only limited polyglutamation in Caco-2 cells exposed to MTX over 24 h.

Functional characterization of PCFT/HCP1 as the molecular entity of the carrier-mediated intestinal folate transport system in the rat model

Proton-coupled folate transporter/heme carrier protein 1 (PCFT/HCP1) has recently been identified as a transporter that mediates the translocation of folates across the cellular membrane by a proton-coupled mechanism and suggested to be the possible molecular entity of the carrier-mediated intestinal folate transport system. To further clarify its role in intestinal folate transport, we examined the functional characteristics of rat PCFT/HCP1 (rPCFT/HCP1) expressed in Xenopus laevis oocytes and compared with those of the carrier-mediated folate transport system in the rat small intestine evaluated by using the everted tissue sacs. rPCFT/HCP1 was demonstrated to transport folate and methotrexate more efficiently at lower acidic pH and, as evaluated at pH 5.5, with smaller Michaelis constant (K(m)) for the former (2.4 microM) than for the latter (5.7 microM), indicating its characteristic as a proton-coupled folate transporter that favors folate than methotrexate as substrate. rPCFT/HCP1-mediated folate transport was found to be inhibited by several but limited anionic compounds, such as sulfobromophthalein and sulfasalazine. All these characteristics of rPCFT/HCP1 were in agreement with those of carrier-mediated intestinal folate transport system, of which the K(m) values were 1.2 and 5.8 microM for folate and methotrexate, respectively, in the rat small intestine. Furthermore, the distribution profile of the folate transport system activity along the intestinal tract was in agreement with that of rPCFT/HCP1 mRNA. This study is the first to clone rPCFT/HCP1, and we successfully provided several lines of evidence that indicate its role as the molecular entity of the intestinal folate transport system.